The problem of controlling static charge is well known in the field of photography. Static charging may occur due to various factors in the manufacture, finishing, and use of photographic elements. The accumulation of static charges can result in fog patterns in photographic emulsions, various coating imperfections such as mottle patterns and repellency spots, dirt and dust attraction which may result in the formation of "pinholes" in processed films, and a variety of handling and conveyance problems.
To overcome the problem of accumulation of static charges it is conventional practice to provide an antistatic layer in photographic elements. Many antistatic agents have been utilized for this purpose. For example, an antistatic layer comprising an alkali metal salt of a copolymer of styrene and styrylundecanoic acid is disclosed in U.S. Pat. No. 3,033,679. Photographic films having a metal halide, such as sodium chloride or potassium chloride, as the conducting material, in a hardened polyvinyl alcohol binder are described in U.S. Pat. No. 3,437,484. In U.S. Pat. No. 3,525,621, the antistatic layer is comprised of colloidal silica and an organic antistatic agent, such as an alkali metal salt of an alkylaryl polyether sulfonate, an alkali metal salt of an arylsulfonic acid, or an alkali metal salt of a polymeric carboxylic acid. An antistatic layer comprised of an anionic film forming polyelectrolyte, colloidal silica and a polyalkylene oxide is disclosed in U.S. Pat. No. 3,630,740. In U.S. Pat. No. 3,681,070, an antistatic layer is described in which the antistatic agent is a copolymer of styrene and styrene sulfonic acid. U.S. Pat. No. 4,542,095 describes antistatic compositions comprising a binder, a nonionic surface-active polymer having polymerized alkylene oxide monomers and an alkali metal salt. In U.S. Pat. No. 4,916,011, an antistatic layer comprising a styrene sulfonate-maleic acid copolymer, a latex binder, and an alkyl-substituted trifunctional aziridine crosslinking agent is disclosed.
It is known to prepare an antistatic layer from a composition comprising a vanadium pentoxide colloidal gel as described, for example, in U.S. Pat. No. 4,203,769. Antistatic layers containing vanadium pentoxide provide excellent protection against static and are highly advantageous in that they have excellent transparency and their performance is not significantly dependent on humidity. The excellent performance of these antistatic layers results from the particular morphology of this material. The colloidal vanadium pentoxide gel consists of entangled, high aspect ratio, flat ribbons about 50-100 angstroms wide, about 10 angstroms thick and about 1000-10000 angstroms long. Low surface resistivities can be obtained with very low vanadium pentoxide coverages as a result of this high aspect ratio morphology.
Typically, the vanadium pentoxide is coated in a polymeric binder to improve adhesion to adjacent layers and to improve the durability of the antistatic layer. Polyesterionomer dispersions, which are preferred binders because they exhibit excellent film-forming properties and compatibility with vanadium pentoxide, have recently been disclosed for use in aqueous-based, vanadium pentoxide-containing antistat coating formulations. For example, an element comprising a support, at least one imaging layer, and an antistat layer comprising vanadium pentoxide in a polyesterionomer binder containing carboxyl groups, alkali metal carboxylate groups, sulfonic acid groups, or alkali metal sulfonate groups is described in U.S. Pat. No. 5,360,706. An imaging element for use in electrostatography containing an electroconductive layer comprising vanadium pentoxide dispersed in a polymer binder such as a polyesterionomer dispersion is described in U.S. Pat. No. 5,380,584. Antistatic layers containing vanadium pentoxide in sulfopolymer binders, including sulfopolyesters are described in U.S. Pat. Nos. 5,203,884, 5,322,761, 5,372,985, 5,407,603, 5,424,269, 5,427,835, 5,439,785, and 5,468,498.
It is known to overcoat a vanadium pentoxide antistatic layer with a hydrophobic, protective overcoat or barrier layer to prevent the dissolution of the antistatic material in film processing solutions which would otherwise result in a diminution of the antistatic properties. Such barrier layers are described in U.S. Pat. Nos. 5,006,451 and 5,221,598, for example. However, the need to overcoat a vanadium pentoxide layer with a barrier layer has several potential disadvantages. These include the following:
(1) An additional layer must be coated and dried. This increases both manufacturing complexity and cost.
(2) In-some critical applications it may be desirable to have the antistatic layer serve as the surface layer since there may be some loss in the effectiveness of the antistat properties when the conductive layer is buried below an electrically-insulating, barrier layer.
(3) When a protective overcoat for the antistatic layer is desired to provide other properties such as friction control, abrasion resistance, ferrotyping and blocking resistance, etc., the choice of materials for the protective overcoat is limited by the need for the layer to also serve as a barrier layer.
(4) It may be desirable to overcoat the antistatic layer with a water (or film processing solution) permeable, hydrophilic layer such as a curl control layer or pelloid layer. However, such permeable layers can not prevent the dissolution of the vanadium pentoxide.
As mentioned hereinabove, it is known from U.S. Pat. No. 5,360,706 to form an antistatic layer of an imaging element by dispersing electrically-conductive colloidal vanadium pentoxide in a polyesterionomer binder. As described in the '706 patent, use of a polyesterionomer binder provides improved coating solution stability and enhanced interlayer adhesion. However, a hydrophobic protective overcoat is required to obtain process-surviving antistatic protection.
It is known from U.S. Pat. No. 5,427,835 to form antistatic layers from an aqueous-based mixture comprising colloidal vanadium pentoxide and a dispersed sulfonated polymer but such mixtures lack the degree of process-surviving capability desired in imaging elements.
It is known from. U.S. Pat. No. 5,096,975 to form an antistatic layer of an imaging element from a combination of a copolymer of vinylbenzene sulfonic acid and a methoxyalkylmelamine crosslinking agent. As described in the '975 patent, this combination provides process-surviving antistatic protection. However, the conductive properties are humidity dependent.
For many imaging elements, and especially photographic films and papers, it is important that the electrically-conductive properties be essentially independent of humidity. Thus, for example, photothermographic elements are typically developed by heating in a high temperature processing chamber in which relative humidity is very low and a layer which is electrically-conductive only under conditions of high relative humidity is entirely unsatisfactory. Equally important for many imaging elements is the requirement that the electrical conductivity be process-surviving. Thus, for example, the electrically-conductive layer must not dissolve in a developing solution or other solutions employed in processing the imaging element. If the electrically-conductive layer is an outermost layer, it is also highly desirable that it resist softening or becoming tacky as a result of contact with processing baths as a soft and tacky surface is easily damaged and prone to dirt pickup in processing equipment.
It is toward the objective of providing an improved electrically-conductive layer for imaging elements whose conductivity is both process-surviving and humidity-independent that the present invention is directed.